1. Field of Art
The disclosure relates generally to earth boring bits used to drill a borehole for applications including the recovery of oil, gas or minerals, mining, blast holes, water wells and construction projects. More particularly, the disclosure relates to percussion hammer drill bits.
2. Background of Related Art
In percussion or hammer drilling operations, a drill bit mounted to the lower end of a drill string simultaneously rotates and impacts the earth in a cyclic fashion to crush, break, and loosen formation material. In such operations, the mechanism for penetrating the earthen formation is of an impacting nature, rather than shearing. The impacting and rotating hammer bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole created will have a diameter generally equal to the diameter or “gage” of the drill bit.
A typical percussion drilling assembly is connected to the lower end of a rotatable drill string and includes a downhole piston-cylinder assembly coupled to the hammer bit. The impact force is generated by the downhole piston-cylinder assembly and transferred to the hammer bit via a driver sub. To promote efficient penetration by the hammer bit, the bit is “indexed” to fresh earthen formations for each subsequent impact. Indexing is achieved by rotating the hammer bit a slight amount between each impact of the bit with the earth. The simultaneous rotation and impacting of the hammer bit is accomplished by rotating the drill string and incorporating longitudinal splines which key the hammer bit body to a cylindrical sleeve (commonly known as the driver sub or chuck) at the bottom of the percussion drilling assembly. The hammer bit is rotated through engagement of a series of splines on the bit and driver sub that allow axial sliding between the components but do not allow significant rotational displacement between the hammer assembly and bit. As a result, the drill string rotation is transferred to the hammer bit itself. Rotary motion of the drill string may be powered by a rotary table typically mounted on the rig platform or top drive head mounted on the derrick.
Without indexing, the cutting structure extending from the lower face of the hammer bit may have a tendency to undesirably impact the same portion of the earth as the previous impact. Experience has demonstrated that for an eight inch hammer bit, a rotational speed of approximately 20 rpm and an impact frequency of 1600 bpm (beats per minute) typically result in relatively efficient drilling operations. This rotational speed translates to an angular displacement of approximately 5 to 10 degrees per impact of the bit against the rock formation.
The hammer bit body may be generally described as cylindrical in shape and includes a radially outer skirt surface aligned with or slightly recessed from the borehole sidewall and a bottomhole facing cutting face. The earth disintegrating action of the hammer bit is enhanced by providing a plurality of cutting elements that extend from the cutting face of the bit for engaging and breaking up the formation. The cutting elements are typically inserts formed of a superhard or ultrahard material, such as polycrystalline diamond (PCD) coated tungsten carbide and sintered tungsten carbide, that are press fit into undersized apertures in bit face. During drilling operations with the hammer bit, the borehole is formed as the impact and indexing of the drill bit, and thus cutting elements, break off chips of formation material which are continuously cleared from the bit path by pressurized air pumped downwardly through ports in the face of the bit.
In oil and gas drilling, the cost of drilling a borehole is very high, and is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits which will drill faster and longer, and which are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed depends upon its rate of penetration (“ROP”), as well as its durability. The form and positioning of the cutting elements upon the bit face greatly impact hammer bit durability and ROP, and thus are critical to the success of a particular bit design.
To assist in maintaining the gage of a borehole, conventional hammer bits typically employ a gage row of hard metal inserts along the gage surface of the cutting face. The gage surface generally represents the radially outermost portion of the bit face, and is configured and positioned to cut the corner of the borehole as the hammer bit impacts the formation. In this position, the gage cutting elements are generally required to cut both a portion of the borehole bottom and sidewall. The lower surface of the gage cutting elements engages the borehole bottom, while the radially outermost surface scrapes the sidewall of the borehole. Excessive wear of the gage cutting elements can lead to an undergage borehole, decreased ROP, increased loading on the other cutting elements on the bit, and may ultimately lead to bit failure.
Moving radially inward from the gage row, conventional hammer bits also typically include an “adjacent to gage” row. Cutting elements in the adjacent to gage row are mounted radially inside the gage row and are orientated and sized in such a manner so as to cut the borehole bottom. In addition, conventional bits typically include a number of additional rows of cutting elements that are located on the bit face radially inward from the adjacent to gage row. These cutting elements are sized and configured for cutting the bottom of the borehole and are typically described as inner row cutting elements and, as used herein, may be described as bottomhole cutting elements.
As previously described, during drilling operations, the hammer bit impacts the formation and indexes in a cyclical fashion. As the hammer bit rotates, the cutting elements extending from the bit face slide across the borehole bottom. Since gage cutting elements are the radially outermost cutting elements on the bit face, they experience greater linear velocities and travel (slide) across a greater distance of the borehole bottom when the hammer bit is indexed as compared to other cutting elements on the bit face. Due to the combination of impacting the borehole bottom, scraping the borehole sidewall, and sliding across the borehole bottom during indexing, gage cutting elements are typically the most susceptible to premature damage and failure as compared to the other cutting elements on the hammer bit.
Increasing ROP while simultaneously increasing the service life of the drill bit will decrease drilling time and allow valuable oil and gas to be recovered more economically. Accordingly, cutting element orientation and placement along the cutting face of a hammer bit that enable increased ROP and longer bit life would be particularly desirable.